Ocean warming conundrum

Not by much, but spread over the vast depths of the deep the change is significant, adding to sea level rise and possibly heralding even greater impacts for mankind and the planet.

While scientists aren’t yet certain if the warming is caused by climate change, they are scrambling to learn more about what’s going on.

This is because the layer starting roughly 2km (one mile) from the surface makes up about half the world’s oceans and plays a key role in regulating the planet’s climate.

“A decade or so ago we had this picture in our minds that deep oceans were pretty stable and that things didn’t change much there,” said oceanographer Steve Rintoul of Australia’s state-backed science and research body CSIRO.

“What’s changed in the last decade is that we’ve started to accumulate enough measurements to show there are widespread changes happening in the deep ocean. And those include really remarkably widespread warming of the deepest layers of the ocean,” he told reporters from Hobart, Tasmania.

Water expands as it gets warmer and this, along with the melting of glaciers and ice caps, is a major force behind rising sea levels.

Seas, on average, are rising at a rate of 3mm a year but some studies suggest they could rise by up to a metre by 2100, inundating low-lying coastlines.

“The heat storage aspect is important because over the past 50 years, about 90 percent of the extra heat stored by the earth is now found in the ocean,” said Rintoul. The deep ocean takes up 10 to 20 percent of this.

Scientists say that extra heat is being trapped by greenhouse gases released by agriculture, deforestation and the burning of fossil fuels.

Warmer waters
The greatest warming of the deep oceans has been recorded near Antarctica and the North Atlantic.

These are the two regions where very cold, salty water sinks from the surface to the depths in a motion that helps drive a global circulation of ocean currents that regulates the climate, for example by giving northern Europe its mild weather.

The water that sinks off parts of Antarctica heads north into different ocean basins as it branches out. It can take centuries to make its way back to the surface.

“We’re seeing warming. We’ve only seen this pattern for a decade or two now,” said Gregory Johnson, an oceanographer of the US National Oceanic and Atmospheric Administration.

He pointed to the difficulties of taking measurements in the crushing depths, which have limited scientists to taking samples every decade on costly voyages that transect an area of ocean.

“When we go out and do these measurements, we go out and stop the ship and lower the instrument down to the bottom and bring it back. It’s sort of like going across the ocean at a slow jog because you spend more than half your time stopped and sampling”.

He said the observed warming rate for the deep layers of the Southern Ocean, between Australia and Antarctica, was about 0.03 degrees Celsius per decade.

“It seems very small but it’s actually a huge amount of energy uptake. Compared with mankind’s global energy consumption rate, it’s three times that rate going into the deep ocean,” he told reporters from Seattle.

“That’s about four Hiroshima bombs every five seconds, or five hairdriers for all 6.8 billion people on the planet going continuously,” he said.

Carbon impact
Some areas, such as the Southern Ocean, have been sampled more than others.

And what scientists have found is worrying.

The water sinking off Antarctica is becoming fresher and therefore less dense, though it’s unclear if this will lead to long-term changes in the speed of deep ocean currents.

Changes in wind patterns are also causing more deep, carbon-rich water to come to the surface.

The oceans are a major carbon “sink”, soaking up large amounts of the main greenhouse gas carbon dioxide, including about a quarter of all the CO2 emitted by human activity.

Oceans store about 50 times the CO2 in the atmosphere. And most of this is stored in intermediate and deep ocean waters.

“There are huge amounts of carbon stored in those waters below the 2,000 metre mark,” said Bernadette Sloyan of the CSIRO’s Marine and Atmospheric Research division in Hobart.

“And changing the temperature changes the ability of the ocean to hold and store that carbon as a reservoir,” she said.

Mankind’s fossil fuel emissions are the equivalent of about six billion tonnes of carbon annually, a fraction of the estimated 38 to 40 trillion tonnes of carbon stored in the intermediate and deep ocean layers.

At present, while the ocean naturally releases carbon dioxide gas in upwelling currents off Antarctica and in parts of the tropics, the world’s oceans overall soak up more than they emit.

Putting the heat on
But scientists say that could change.

“The changes we project will happen in the Southern Ocean will tend to make the ocean less effective at storing CO2,” said Rintoul.

The deep oceans are also a major source of nutrients, such as iron, that ocean ecosystems need to survive.

“Changes in the circulation of the ocean and deep ocean and how it interacts with the upper ocean will have significant impacts on bringing nutrients back up to allow ecosystems to keep thriving,” Sloyan said.

For now, scientists are trying to speed up measurements to figure out if mankind has woken up a monster in the deep.

Rintoul and Johnson said more study was needed to pin down any direct climate change connection.

“At the moment we can’t really say that the pattern of deep warming that we see is a signal of human-caused climate change,” said Rintoul.

“And the reason we can’t say that is partly because we only have a few decades of observations and also because we don’t really understand the processes that control variability in the deep ocean as well,” he said.

Self-propelled gliders being developed to take deep water samples will help. Engineers are also working to expand a network of floats, which can presently sink to 2,000 metres to take data readings, with new models that can go even deeper.

“It’s said we know more about Mars than we know about the deep ocean. It’s absolutely true,” Sloyan said.